Recent Developments on Detection of Zn2+ Ions Using Schiff Base Probes

limits of these probes demonstrated that the Schiff base probe 34 possessing two benzothiazole moieties exhibits the lowest detection limit (0.00028 µM) indicating it to be the lead compound in the determination of Zn2+ ions in near future


Introduction
Zinc is a crucial trace element for the human health.It is essential for maintaining our physical and mental well-being as well as the growth of our immunity.. [1,2,3,4] Intake of Zn 2+ ions in excess and deficient quantities can cause several diseases.[5,6,7,8,9,10,11,12] Deficiency of Zn 2+ ions in human body are known to create the onset of Alzheimer's disease.[13,14,15] While, its excess can hamper the bacteriostatic power of scavenger cells which reduces the immune functions of the body resulting in the decreased capability to fight with diseases.[16,17,18] In addition, Zn 2+ is crucial to plant metabolism because it influences the growth and development of plants.[19] However, excessive concentrations of zinc in the environment seriously harm the environment.Because of these difficulties, it is essential to discover sensitive and specific ways to identify Zn 2+ ions that are simple to use and provide a swift response.[20,21,22] Currently, a number of techniques are available for the metal ion recognition, such as electrochemical, [23] neutron activation analysis, [24] high-performance liquid chromatography (HPLC), [25] atomic absorption and emission spectroscopy, [26,27] mass spectrometry, [28] anodic striping voltammetry, [29] electrothermal atomic absorption spectrometry, [30] capillary electrophoresis [31] and inductively coupled plasma-mass spectrometry.[32] Although these techniques have good selectivity, sensitivity and rapid This article is protected by copyright.All rights reserved.

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response, they have some limitations or drawbacks as well such as these techniques are too expensive, require complicated instrumentation handling along with a large amount of sample pre-treatment.[33,34] Therefore, highly sensitive, selective and efficient method is needed for the detection of Zn 2+ ions at trace levels.So, two types of promising chemosensors; namely, colorimetric and fluorometric chemosensors are available and are under vigorous research for the detection of Zn 2+ ions majority of which are based on the Schiff bases.
Schiff bases have a spectacular coordinating affinity with the zinc ion complex and are recognized as exceptional chelating ligands.[35,36] Because of their extraordinary properties, schiff bases are currently employed as efficient chemosensors to identify various cations and anions and electron-deficient substances like nitro-aromatics.[37,38] Colorimetric sensors are a type of chemical sensors that detect analytes by producing visible color change.Colorimetric sensing [39] offers selective and sensitive detection of various kinds of species such as metal cations, anions, drugs, toxic waste, biomolecules and organic dyes.
Without the use of sophisticated devices, the colorimetric chemosensor provides reproducible, visible to the unaided eye, low-cost, effective, fast, and appropriate detection of environmental and biological toxicities.[40] Because of internal charge transfer (ICT), which is brought on by interactions between the chromophore and the analyte's hydrogen atoms, there is a considerable shift in the chromophore units' ability to absorb light, [41] provides them with high prospective of analyte detection methods.
The fluorometric chemosensors are the chemical breed, which can be easily determine the various types of analytes When interacting with a different species of analytes [42], such as biological scaffolds, neutral molecules, anions and cations, fluorometric chemosensors can detect a specific substance based on its ability to emit fluorescence, due to its fluorescent characteristics that can be undergo photophysical changes.Fluorescent sensors are frequently utilized as analyte determining probes in a extensive variety of biological application due to the fact that they exhibit a change in emission spectra, wavelength, and lifetime decay during the molecule recognition.[43] While increasing the wavelength, a probe is deactivated from the excited singlet state to the ground state, produced fluorescence with a typical lifespan decay of 10-9 s. [44] There are two types of phenomena involves in fluorescence, (1) Quenching and (2) Enhancement.Fluorescence enhancement is the process of increasing the ability of fluorescence emitted by the fluorescent molecules.Whereas, fluorescence quenching is the process which decreases the emission intensity of the fluorescent probes.[45] Most of the This article is protected by copyright.All rights reserved.

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reported fluorometric and colorimetric probes are the organic skeleton including Schiff base moieties., because of their oversimplified synthetic route and extraordinary coordination binding sites with the ions.[46] In this article, authors reported the predesigned Schiff base chemosensors and synthesized by different research groups since 2018 to encapsulate zinc metal ions selectively and specifically using various spectrophotometric techniques.
It is believed that these Schiff base probes will provide crucial information towards future chemosensors that are more efficient.Table 1 demonstrates the recognition of Zn 2+ ions having lowest detection limits with the comparison of Schiff base probes.

Schiff base probes for Zn 2+ recognition
To the determination of Zn 2+ ions, a fluorescent Schiff base sensor 1 (Figure 1) has been synthesized by Gau Xu and his research group.[47] Xu et.al. reported a hydrazone Schiff base based chemosensor which was further characterized by various spectroscopic techniques.The sensor shows weak fluorescence alone, but with the incorporation of Zn 2+ ions, the intensity of fluorescence enhanced rapidly and exhibited an intense yellow-green emission.The binding stoichiometry ratio of sensor 1 observed to be 1:1, as confirmed by the jobs plot, and the binding constant was found to be 184 M -1 .The limit of determination was found to be 112 µM and also designed a test paper strip for rapid detection of Zn 2+ ions.
A responsive multi-stimulated organic fluorescent compound 2 (Figure 1) synthesized by Francis Joy and her research group [48] for the detection of Zn 2+ ions in water-based solution.
This hydrazone-based Schiff base turn-on chemosensor was highly sensitive and selective in nature towards the Zn 2+ ion.The chemosensor was investigated and showed a remarkable increase in emission intensity towards the Zn 2+ cation in water-based solution while in case of other metal cations, the intensity drastically decreased.The stability constant was found to be 4.587×10 3 and detection limit was found to be 9.315 µM.On the other hand, researchers also made a film composite of synthetic compound to sense Zn 2+ ions in solid state.In addition, to determination of Zn 2+ ions in cell imaging taken from the nervous cells of rats using this fluorescent sensor 2 have also been done.
Li et al. [49] Reported, a novel naphthalene-based Schiff base probe 3 (Figure 1) for the detection of Zn 2+ ions.This synthesized probe showed a turn-on fluorescent nature towards Zn 2+ ions.The probe shows good sensitivity and selectivity towards Zn 2+ ions in DMF/H2O solution.Which ascribes chelation enhanced fluorescence mechanism (CHEF) and the probe This article is protected by copyright.All rights reserved.

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shows absorbance band at 280 and 380 nm after adding Zn 2+ ions into the solution.The absorbance band shifted towards a red shift at 357 nm.This phenomenon shows the interaction between the probe and Zn 2+ ions.The emission intensity shown by the probe -Zn 2+ ions at 425 nm.The coordination ratio observed to be 3:1 between the probe and Zn 2+ ion, determined with the help of jobs plot.The lower limit of detection was 8.94×10 -8 M and response time was just a few seconds.
Yang and co-workers [50] have synthesized Dicyanisophorone based fluorescent probe 4 (Figure 1) for the detection of Zn 2+ ions.This synthesized probe was rapidly combined with Zn 2+ ions and shows an absorbance band at 500 nm.At the stage of the combination of the probe and Zn 2+ ions color changes were observed from light yellow to orange.Meanwhile, the intensity of fluorescence shows a large stock shift of 179 nm.The response time was found to be just less than 30 seconds.The limit of detection was calculated to be 4.8 nM, which was suitable for the detection of Zn 2+ ions in actual water samples.In addition, a sharp and clear image has been obtained in A549 cells.
This article is protected by copyright.All rights reserved.To detect the Zn 2+ ions, a Schiff base scaffold chemosensor 5 (Figure 2) has been developed by Malini Nelson and her research group.[51] This synthesized chemosensor has been described with the help of different spectroscopic techniques.This sensor showed high selectivity and responsiveness toward Zn 2+ ions in DMSO/H2O solution.Sensor shows fluorogenic response with Zn 2+ ions and shows weak emission to bluish green fluorescence accompanied by turn-on and off processes.The limit of detection and binding constant for the Zn 2+ ions were calculated to be 2.23 nM and 2.3115×10 3 M -1 .In addition, the quantum yield This article is protected by copyright.All rights reserved.

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was calculated for the sensor and complex was determined 0.008 and 0.8, respectively.As well, water samples, swab tests and test strips also used to demonstrate the determination of Zn 2+ ions.This sensor also compatible with detection of Zn 2+ ions in A549 cells imaging.
A fluorescent probe 6 (Figure 2) has been designed by Yu and his group [52] for the determination of Zn 2+ ions in an aqueous solution.A rapid increase in the fluorescent emission intensity was observed with Zn 2+ ions in the solution.It was very specific towards the Zn 2+ ions.The molar binding ratio was 2:1 between the probe 6 and the Zn 2+ ion.The limit of detection was calculated to be 2.43×10 -5 M.Not any kind of cytotoxicity on HeLa cells.
A benzothiazole-based fluorescent chemosensor 7 (Figure 2) has been reported by Li et al. [53] Using fluorescence spectroscopy, this sensing capacity was identified.There was a high degree of selectivity in the probe towards Zn 2+ ions.This sensor works well with the wide range of pH 7-10.The sensor's stoichiometry ratio 1:1, with a detection limit 0.68 µM.The response time for the detection of Zn 2+ ions was just 30 seconds.This article is protected by copyright.All rights reserved.

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A bidentate Schiff base ligand 8 (Figure 3) has been synthesized for the determination of Zn 2+ ion in water by Viviana et al. [54] The structure of synthesized ligand 8 has been characterized with the help of FTIR, NMR and its optical characteristics by the UV-visible absorbance spectroscopy and photoluminescence measurements.In case of UV-vis spectroscopy, the synthesized probe shows two intense absorption bands that were observed at 318 nm and 380 nm due to the π-π* electronic transition of the conjugated aromatic ring and n-π* electronic transition of the amine group, respectively.While in case of fluorescence studies, luminescent quenching of ligand was detected on the addition of Zn 2+ ion in the ligand.Also, a simple modified screen-print carbon electrode (SPCEs) was also used to determine the micro detection limit of the synthesized ligand with Zn 2+ ion.For the detection of Zn 2+ ion, good linearity, high sensitivity, and selectivity of 50.1 mV/dec were established.The response time was calculated to be 10 sec.
Successfully synthesized a phenolphthalein derivative chemosensor 9 (Figure 3) for the detection of Zn 2+ ions by Aydin and Meliha Kutluca Alici.[55] This sensor 9 could selectively monitor Zn 2+ ions easily and impart color change in EtOH:HEPES buffer solution.On the addition of Zn 2+ ions into the solution, sensor 9 showed a high enhancement in the fluorescence intensity.The stoichiometry ratio was found to be 1:2 between the sensor 9 and Zn 2+ ions.The detection limit was calculated to be 4.21 nm, determined with the help of jobs plot.This chemosensor also detects Al 3+ ions.
Sahu et al. [56] Synthesized a turn-on fluorescence moiety 10 (Figure 3) based on the isoindole -imidazole scaffold for the detection of Zn 2+ ion.This Schiff base chemosensor was synthesized and characterized with the help of various spectroscopic techniques such as HRMS, HMR and FTIR etc.The optical properties have been investigated with the help of UVvisible spectroscopic and fluorescence techniques.From the UV-vis studies, the absorption peak of moiety was at 405 nm and 480 nm.This moiety changes its color from colorless to pale yellow in the presence of Zn 2+ ions.Gradual increase in the concentration of Zn 2+ ions in the solution increased the absorbance peak at 419 nm, that shows the interaction of Zn 2+ ions with probe 10.The moiety shows weak fluorescence intensity with a quantum yield of 0.036, while the addition of Zn 2+ ions in the solution resulted in the fluorescence intensity enhancement by 19-folds with a quantum yield of 0.69.The detection limit was found to be 0.073 µM.The titration and jobs plot indicates the formation of 1:1 moiety: Zn 2+ bindings.The binding constant was found to be 3×10 4 M -1 .
This article is protected by copyright.All rights reserved.Han and his research group [57] developed a quinoline based supramolecular gel 11 (Figure 4) for the selective detection of Zn 2+ ions.This Schiff base probe 11 forms organo gels with five This article is protected by copyright.All rights reserved.

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different organic solvents such as acetone, DMSO, DMF, ACN, EtOH and 1,4-dioxane.This assembly was analyzed by field emission scanning electron microscopy (FESEM), UV-visible, fluorescence spectroscopy, FTIR, XRD and water content angles.Under non-covalent interactions, self-assembling takes place in nanofiber structures.Weak fluorescence intensity was shown by the gelator group on the addition of Zn 2+ ion into the solution.The sensitivity and selectivity were also relatively high towards Zn 2+ ions.This shows an enhancement in the fluorescence intensity.The emission intensity increased from 423 nm to 545 nm, and the limit of detection was found to be 5.5 µM.
Developed a turn-on fluorescence Schiff base probe 12 (Figure 4) based on 2-hydroxy-1naphthylaldehyde for the recognition of Zn 2+ ions in the real water samples by Mu et al. [58] The probe was detected with the help of absorption spectroscopy.This probe shows three absorption bands at 262, 325 and 362 nm, respectively.A gradual decrease in the absorbance band was observed with the addition of Zn 2+ in the solution.New bands appeared at 337 nm and 428 nm, and color change observed was colorless to yellow.Four isosbestic points were observed at 284, 333, 339 and 387 nm, that indicate the formation of a new complex between the probe and the Zn 2+ ion.The emission intensity was enhanced by the addition of Zn 2+ ion in the solution.The complexation constant was found to be 1.13×10 5 M -1 .The HOMO-LUMO energy gap between the probe and Zn 2+ was 1.49 eV.The stoichiometry ratio was 1:1 between the probe and Zn 2+ .Moreover, the cell imaging fluorescence experiment demonstrates trace Zn 2+ in living cells with low cytotoxicity.
A visible colorimetric probe 13 (Figure 4) has been designed and synthesized by Venkatesen et al. [59] Schiff base condensation reaction has been done using this diaminomaleonitrile derivative to sense Zn 2+ ions selectively.The sensing ability of this probe was successfully investigated with the help of visual and UV-vis techniques.The absorption band appeared at 595 nm with molar absorptivity 14578 LM -1 cm -1 .This arises due to the metal-to-ligand charge transfer phenomenon (MLCT).The response of the ligand was dependent on the pH of the solution, complex formation easily takes place within the range of pH 8-11.The stoichiometry and binding constant calculated with the help of B-H plots were 1:1 and 2.63×10 3 M -1 .The HOMO-LUMO energy gap between the probe and complex was found to be 2.23 and 2.63 eV, respectively.The points of reference for coordination were (N, N, O, O).It was discovered the detection level was found to be 5.8×10 -7 M. Its employment in logic gates is one of its additional beneficial applications.
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A new innovative coumarin-based fluorescent probe 14 (Figure 4) was synthesized and characterized.The fluorescent probe exhibits significant fluorescent quenching in the presence of Zn 2+ ions by Arval et al. [60] The limit of detection was found to be 3.43×10 -9 M for the detection of Zn 2+ ion.The stoichiometry ratio of the probe and Zn 2+ ion, as determined by the help of jobs plot linear function was obtained to be 1:1, and the association constant was calculated as 2.39×10 4 M -1 .No toxic effect was observed on the L929 cells by the MMT method.This article is protected by copyright.All rights reserved.

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Cheng and L. Zheng [61] synthesized two fluorescent probes 15 (Figure 5) for the detection of Zn 2+ ions in real water.These two probes synthesized with the help of salicylaldehyde benzoyl hydrazine using the Schiff base condensation reaction pathway were characterized by various spectroscopic techniques.Probe 15 show weak fluorescence intensity but, on the addition of Zn 2+ ion in solution, a very high enhancement in the intensity of fluorescence was observed even in the presence of many other metal ions.This shows the selectivity and sensitivity of the designed probe 15 towards Zn 2+ metal ions.The calculated molar binding ratio of probe and Zn 2+ ion was 1:1, and the coordinating sites were O and N of the fluorophore.It was also used in the case of bio-imaging after the incubation of ECV304 with probe for 30 min at 37 o C. The red color was emitted from the intracellular area, which indicates that could be used for dynamic imaging and tracing the Zn 2+ ion in living cells.
A coumarin-tagged diprotic scaffold fluorophore 16 (Figure 5) was derived by.K Bhasin and co-workers [62] for the detection of Zn 2+ ion in the semi-aqueous medium.This multi-dentate ligand works well with Zn 2+ ion, and can easily bind with Zn 2+ ions and shows turn-on fluorescence in a semi-aqueous medium with 0.272 nM concentration.The multi-dentate ligand is having triclinic crystal system and P-1 in space group with 3-coordinating site.The molar ratio of the probe and Zn 2+ ion binding was 1:1, and the detection limit was calculated to be 0.272 nM.Its other perspective was in the field of medicine as Zn 2+ ion-binding moieties play a vital role as pharmacophores in certain drugs.
To sense metal ions such as Zn 2+ ions, Juan Li and his group [63] synthesized a probe 17 (Figure 5) based on oxime and salicylaldehyde.This chemosensor performed well in the case of an aqueous medium (DMSO/H2O, V/V=9:1).The selectivity and sensitivity of this probe 17 towards Zn 2+ ions were determined using spectroscopic techniques for fluorescence and UVvis.A weak absorption band was shown by the ligand at 425 nm, but after the addition of Zn 2+ ions in the solution, the absorbance band shifted towards a higher range that was 458 nm, or we can say red-shift was observed with a color change of the solution from colorless to yellow.
The UV-vis characteristics show that the molar ratio of metal and ligand was 1:2, and the association constant was 5.6×10 4 M -1 .While in the case of fluorescence recognition of Zn 2+ ions using probe 17.The emission peak of the probe 17 was observed at 437 nm, and after the addition of Zn 2+ ions, The emission peak was moved up to 502 nm, a higher wavelength.When Zn 2+ ions were introduced to the solution, the intensity increased by 16 times.It was determined that the detection threshold was 2.53×10 -8 M. The HOMO-LUMO energy gap between the probe and complex, which was calculated through the DFT calculations, was found to be 3.44 This article is protected by copyright.All rights reserved.

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eV.This was utilized in various water samples, including drinking water, tap water, and water from a yellow river.For the detection of Zn 2+ ions in water, food supplements, and bio-imaging applications, Aydin et al. [64] designed a new ultra-sensitive sensor using thiazole derivative.This novel fluorescent probe 18 (Figure 5) was excited at 420 nm, and its emission peak was observed at 503 nm in HEPES/MeCN buffer (5/95, v/v, pH= 7.4) media.The quantum yield of the probe increased from 0.073 to 0.78 with Zn 2+ ions.The detection limit and binding constant were calculated to be 1.29 nM and 1.102 ×10 6 M -1 , respectively.The stoichiometric ratio between probe and Zn 2+ ion was 1:1 that was verified by jobs plot.The mechanism of fluorogenic response was understood via photo electron transfer, and further theoretical DFT studies were also done successfully.
This article is protected by copyright.All rights reserved.A solvent-dependent fluorescent probe 19 (Figure 6) for the detection of Zn 2+ ions was developed by Qin et al. [65] By using the Schiff base condensation reaction pathway.This chemosensor 19 was synthesized with the help of 8-hydroxy quinoline group.It was used to determine the Zn 2+ ions in an EtOH solvent with high sensitivity and selectivity.The stoichiometric molar ratio of this sensor was determined to be 1:1.It was found to inhibit the PET process due to the presence of lone pairs on the nitrogen atom of the C=N group resulting in the enhancement of fluorescence intensity.The limit of detection was calculated to be This article is protected by copyright.All rights reserved.

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2.36×10 -7 M for Zn 2+ ion.The DFT calculation was done and verified the binding ratio and HOMO-LUMO energy gap between the sensor and complex.
Zhang et al. [66] designed a fluorescent organic nano-material having well-defined AIE characteristics for a colorimetric chemosensor.This organic fluorescent coumarin-based material has been synthesized for the determination of Zn 2+ ions.This probe 20 (Figure 6) shows AIE aggregation-induced luminescence enhancement properties in this molecular condition in various aqueous conditions.The determination of Zn 2+ ions was carried out using UV-visible and fluorescent multi-mode of the given probe with high sensitivity and selectivity or specificity.The response time of the probe was significantly less with high instability of complex.The limit of detection was observed to be 0.026 µM in an aqueous solution.The probe displayed green fluorescence in solid powder phase that can be used to sense ZnCl2 powder.It was also combined with a paper strip and polymer thin film to sense Zn 2+ ions by color change.It was also used to detect Zn 2+ ion in-vivo in the zebrafish cell imaging.
Mondal and her research group [67] developed a colorimetric and fluorometric triazine-based sensor 21 (Figure 6) for the recognition of Zn +2 ions.This new probe has three different binding sites for three metal ions.It is selectively detecting the Zn 2+ ions from all the sites of the molecule, or we can say all the three cavities of the probe 21.The probe is present in two forms, one was enol-form, and another was keto-form.The selectivity of this probe was observed with the help of UV-visible and emission spectroscopy.The absorbance of this probe was observed at 330 nm and 338 nm, and after the addition of Zn 2+ ions, a new peak appeared at 385 nm with a sharp isosbestic point at 355 nm that indicates the formation of a new complex.The emission peak was observed at 430 nm.After the addition of Zn 2+ ions, a new peak was observed in the emission spectra also at 468 nm with fluorescence intensity enhancement of upto five times.
The coordination ratio of the probe and Zn 2+ ion was 3:1.The limit of detection was calculated to be 1.22×10 -7 and 1.13×10 -7 M for R1 and R2, respectively.
Li et al. [68] Synthesized and designed a novel diaryl-ethene derivative for the detection of Zn 2+ ions.The structure of the synthesized probe 22 (Figure 6) has been characterized by the help of various spectroscopic techniques.The emission spectra displayed excellent sensitivity and selectivity towards Zn 2+ ions with emission intensity enhancement of 154-fold with bright blue fluorescence.1:1 was the stoichiometric ratio of the probe and Zn 2+ ion, as confirmed with the help of the jobs plot.It can also detect phosphate and sulfate via the quenching effect, and its other application is that it could also be used as logic gate.
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A new fluorescent moiety has been synthesized by Mathew and Anandram Sreekant.[69] The Thiophene-dicarbohydrazide based moiety works well as a colorimetric sensor for the detection of Zn 2+ ions.The chemosensor 23 (Figure 7) was observed to be highly emissive in nature when it binds with the Zn 2+ ions and forms complex in DMSO/Water (6:4, v/v) medium.The binding constant was calculated to be 1.15×10 4 M -1 .The detection limit was found to be 1.51×10 -7 M which is well below the permission permissible level of Zn 2+ ion (70 µM) in drinking water by WHO.The binding molar ratio was 1:1 between Zn 2+ ions and probe 23.The reversibility and INHIBIT/IMPLICATION logic gate behavior were also shown for this chemosensor.
An important fluorescent probe 24 (Figure 7) has been designed by Wang and his research group [70] for the detection of Zn 2+ ion in a real water sample.This synthesized probe 24 has very high selectivity and sensitivity towards Zn 2+ ions.An emission spectroscopy technique was used to investigate it, that shows the large stock shift of 110 nM with a change in the concentration of solution of Zn 2+ ions from 0 to 20 µM.The limit of detection was 19.1349 nM and binding constant was calculated to be 3.24×10 4 M -1. and stoichiometric I was one ratio one.This probe was sensitive to a wide pH range.All the interactions of ligand and Zn 2+ ions were verified by the density function theory studies.
Two similar kinds of Zn 2+ ion sensors 25 (Figure 7) have been developed with the help of quinoline derivative by Fan et al. [71] This fluorescent probe was synthesized with the help of Schiff base condensation pathway.These two sensors have been investigated using UV-visible and fluorescence spectroscopy.In case of UV-visible absorption band, these sensors show peaks at 321 nm and 295 nm, respectively.The intensity of these bands decreased gradually while the addition of Zn 2+ ions in the probe solution with the appearance of a new band at 306 nm.Two isosbestic points were also observed at 341nm and 267 nm, that indicates that the stable complex was formed between the sensor and the Zn 2+ ion.The emission band appeared at 450 nm in an ethanolic solution.Significant enhancement in emission intensity was observed with the addition of Zn +2 ions.The molar stoichiometric ratio was 1:1 for the sensor and Zn 2+ ions.The lower value of detection was found to be 3.5×10 -7 M.
A simple synthesis of the probe 26 (Figure 7) for the detection of Zn +2 ion was developed by Jiao et al. [72] This synthesized fluorescent probe was highly sensitive and selective towards Zn 2+ ions that was successfully investigated by emission spectroscopy.The jobs plot and HRMS experiments results show that the probe 26 and Zn 2+ ion binding ratio was 1:1, and the binding constant was calculated as 7.998×10 4 M -1 .The detection limit was observed to be 46 µM.It could also be applied for the determination of Zn 2+ ions in living C6 cells.
This article is protected by copyright.All rights reserved.A fluorometric based chemosensor 27 (Figure 8) that can easily sense Zn 2+ ion in an aqueous medium was established by Kim et al.[73]This sensor was designed with the help of salicylaldehyde and benzyl carbonate in the ethanol solvent using Schiff base condensation manner.To investigate its sensitivity and selectivity, the emission spectroscopy technique was used.The fluorescence emission of the sensor was observed at around 452 nm, but after the addition of Zn 2+ ions into the solution, a significant enhancement of fluorescence intensity was observed.The binding molar ratio of the sensor and Zn 2+ ion was 1:1, as determined by analyzing B-H and jobs plot.The association constant was calculated to be 2.1×10 3 M -1 .The LOD was found to be 1.12 µM that is lower than the permissible value given by WHO.The binding interaction was also explained with the help of DFT calculations.It was also used for the cell imaging of zebrafish.

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Coumarin-based Schiff base fluorescent chemosensors 28 (Figure 8) were designed for the detection of Zn 2+ ion by Fu et al. [74] These probes displayed high selectivity and sensitivity for Zn 2+ ions in a buffered medium (DMF/HEPES) solution.These sensors show good fluorescent response on the addition of Zn 2+ ions into the solution resulting in the enhancement of fluorescence intensity that can be seen in the visible range or by the naked eyes.The detection limit of probe 28A and 28B towards Zn 2+ ion was separately calculated as 1.03×10 -7   M and 1.87×10 -7 M, respectively.These two probes can also detect phosphate anions.
Furthermore, the cell imaging demonstrated that these probes could detect Zn 2+ ions in in-vitro cells.
A Schiff base probe 29 (Figure 8) has been designed for the determination of Zn 2+ ions in acetonitrile/HEPES solution at pH 7.0.By Chang et al. [75] The probe 29 showed high enhancement in fluorescence intensity towards Zn 2+ ion with a color change from faint yellow to blue.The molar stoichiometric ratio between the probe and Zn 2+ ion was 1:2, this resulted in the inhibition of intramolecular charge transfer and chelation-enhanced fluorescence process.The mechanism of bonding was also confirmed by using UV-vis, fluorescence, HRMS and NMR techniques, and theoretical calculations were done using DFT computational studies.
It can be used efficiently to detect Zn 2+ ions in living cells.The limit of detection was calculated to be 8.19×10 -8 M, and the binding constant was found to be 1.75×10 4 M -2 .
This article is protected by copyright.All rights reserved.Lu and his research group [76] successfully designed and synthesized a novel kind of fluorescent moiety 30 (Figure 9) based on the diarylethene and benzohydrazide unit to detect Zn 2+ ions.The fluorescent and photochromic switching properties have been studied in detail by the UV-vis light.The moiety shows very high sensitivity and selectivity towards the Zn 2+ ion in tetrahydrofuran solvent.The fluorescence intensity was enhanced by 84 times on the addition of Zn 2+ ion in solution and 44 nm of blue shift was also observed after the addition of Zn 2+ ions.The molar ratio of sensor 30 and Zn 2+ ion was 1:1 and the limit of detection was found to be 5.60×10 -9 M.

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Ahmed and his research group [77] have developed a probe 31 (Figure 9) with the help of malonitrile for the detection of Zn 2+ ions.To investigate the sensitivity and selectivity of this probe, UV-vis spectroscopic analysis was used that shows an absorbance band at 487 nm.
Further, with the addition of Zn 2+ ions, light brown color was observed in the solution, even in This article is protected by copyright.All rights reserved.

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the presence of other metal ions and amino acids.In the case of fluorescence emission, the enhancement of fluorescence intensity was observed after the addition of Zn 2+ ions into the solution.The response time was just a few minutes only.This probe works well with a wide range of pH 3-13.Cytotoxicity and cell imaging were also done for the living cells with this probe.
This shows metal-to-ligand charge transfer operated with the synthesized probe which was clearly visible by the naked eye.The binding ratio of the probe 32 and Zn 2+ was 1:1 that was confirmed with the help of jobs plot analysis.DFT computational calculations were also done successfully for this probe and its complex with Zn 2+ ions.This probe could also sense F -ions and its one of the essential applications was seen in cell bio-imaging to monitor Zn 2+ ions in HeLa cells and living cell imaging in zebrafish embryos.
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Acridine-based chemosensor 33 (Figure 10) was developed to sense Zn 2+ ions in watery medium by Nunes et al. [79] It has been found that this sensor is extremely selective for Zn 2+ ions when used as a chemosensor for metal ions.With a concentration range of 17.8-600 M, a linear fluorescence enhancement of approximately 230% was attained.Understanding how a stable complex form was made easier by the B-H plot and the Jobs plot, and the stoichiometry ratio of the sensor 33 and Zn 2+ ion was found to be 1:2.The lower value of detection was 5.36 µM.The association constant was obtained to be 2.43×10 14 L 2 mol -2 .
A turn-on fluorescent based chemosensor 34 (Figure 10) was synthesized by Dhivya and his research group [80] for the determination of Zn 2+ ion.The chemosensing behavior of this probe was established with the help of absorption, fluorescence, NMR, HRMS and FTIR spectroscopic methods.This probe selectively detects Zn 2+ ions even in the presence of other metal ions.The probe shows absorbance peaks at 335 and 370 nm, but upon the addition of Zn 2+ ions into the solution, a slight red shift appeared in the region 338 and 385 nm.The probe shows weak emission at 515 nm but enhanced fluorescence intensity was observed on the addition of Zn 2+ ions.The binding constant was calculated to be 9.3×10 7 M -1 .The quantum yield was 0.016 and 0.173 for the probe 34 and complex, respectively.The limit of detection was found to be 0.28 nM.This probe could also detect phosphate anions.
For the detection of Zn 2+ ions in real water samples, Aziz and his research group [81] synthesized a fluorometric probe 35 (Figure 10) and characterized it with the help of different spectroscopic techniques.Weak emission intensity was shown by the probe at 586 nm (quantum yield was 0.026) and further excitation upon the wavelength 360 nm shows enhancement of emission intensity.1:2 stoichiometric ratio was found between the probe 35 and the Zn 2+ ion.The probe has a rapid response time, and its limit of detection was found to be 0.0311µM.The fluorescence cell imaging demonstration reveals that the probe has good cell membrane permeability and can be used for Zn 2+ ion detection in living cells.
Yan et al. [82] Synthesized a fluorescent probe 36 (Figure 10) to detect Zn 2+ ion based on aggregation-induced emission using picolyl-amine.This probe 36 showed very high selectivity towards Zn 2+ ions and displayed good stock shift (100 nm) after the addition of Zn 2+ ions to the probe solution.The color change was visible by the naked eyes.The detection limit was observed as 1.3×10 -7 M. Low toxicity and excellent permeability of this probe were beneficial for its use in living cell imaging.
This article is protected by copyright.All rights reserved.The development of a Schiff base probe 37 (Figure 11) for the determination of Zn 2+ ion based on the naphthyl group was established by Li et al. [83] This probe 37 displayed its absorbance band at 353 nm and 367 nm in ethanol.Upon the addition of Zn 2+ ion in the solution, its absorbance was decreased, or we can say a blue shift was observed with an isosbestic point at 376 nm that tells us about the formation of new interacted species.In emission intensity, almost no fluorescence emission was observed while adding Zn 2+ ions in the solution resulted in a This article is protected by copyright.All rights reserved.

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sharp and clear enhancement in fluorescence intensity (63-fold).The stoichiometric ratio of the given complex was 2:1, as determined with the help of the jobs plot.The limit of detection was found to be 7.52 M.
Yun et al. [84] reported, a novel benzimidazole-based fluorescence moiety 38 (Figure 11) for the recognition of Zn 2+ ions in an aqueous medium.This moiety shows a turn-on fluorescence effect for the detection of Zn 2+ ions.The sensor showed weak fluorescence, but upon the addition of Zn 2+ ions into the solution, a remarkable emission intensity shift was observed.The quantum yield was calculated as 0.0012 and 0.0122 for the moiety and the complex.The limit of detection was calculated to be 2.26 µM that was below than the WHO permissible limits.
This could be recycled with EDTA.This moiety was also used in case of cell imaging to detect Zn 2+ ions in zebrafish embryos.
This article is protected by copyright.All rights reserved.A fluorescent chemosensor 39 (Figure 12) for the detection of Zn 2+ ion has been designed by Yang Li and his research group [85] based on diarylethene hybrids.The probe acts as a turn-on fluorescence chemosensor to detect Zn 2+ ions in MeOH.The chemosensor has been observed as highly selective and sensitive in nature over the other metal ions.This chemosensor shows weak fluorescence emission alone, but after the addition of Zn 2+ ions into the solution, a new strong fluorescence peak was observed with high intensity (105-fold) and showed a color change in solution from dark to bright yellow.The detection limit was 13.4 nm.This sensor was used to design logic gate and could also be used in case of real water analysis to detect Zn 2+ ion.

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Chae et al. [86] Have developed a fluorometric sensor 40 (Figure 12) for the determination of Zn 2+ ions in an aqueous solution.Fluorescent titration has been done to investigate the sensitivity and selectivity of the sensor.On the addition of Zn 2+ ions into the solution, there was a rapid increase in the fluorescence intensity.The quantum yield was calculated to be 0.0083 and 0.0387 for the Sensor and complex, respectively.The interaction behaviour between the sensor and Zn 2+ ion was established with the help of UV-visible titration, which showed absorbance bands at 300 nm and 350 nm with two isosbestic points at 277 and 330 nm, respectively.The limit of detection was calculated to be 0.27 µM.It could also apply to living cell imaging in HeLa cell imaging in zebrafish.
A water-soluble fluorescence sensor 41 (Figure 12) has been synthesized based on the Benzoimidazolyl-pyridine by Jiang and research group [87] for the detection of Zn 2+ ions and picric acid using cascade mechanism.To investigate the sensing ability of sensor 41 with the help of UV-visible and fluorometric analysis has been done.When Zn 2+ ions were added to the solution, fluorescence intensity quickly increased to 415 nm and the color changed from colorless to blue.The emission band of the sensor alone had a wavelength of 383 nm.A 1.83 ×10 -7 M limit of detection was discovered.
This article is protected by copyright.All rights reserved.M and his group [88] developed a pyridine containing Schiff base probe 42 (Figure 13) for the detection of Zn 2+ ions.This multidentate ligand was determined with the help of absorbance and emission spectroscopy, and was found to be highly sensitive and selective in nature with rapid response time.The quantum yield was calculated to be 1.60% of the complex.The stoichiometric ratio of the ligand 42 and complex was 1:1, and the lower limit of detection value was 7.2 µM.This article is protected by copyright.All rights reserved.

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A chemosensor 43 (Figure 13) based on 3-aminobenzofuran carboxamide Schiff base reaction pathway has been synthesized by Gao et al. [89] The chemosensor has good sensing ability towards Zn 2+ ions which were analyzed by several spectroscopic tools.This chemosensor binds with the Zn 2+ ion selectively and shows enhancement in fluorescence emission intensity.Color change was observed from dark to bright orange that can be seen by naked eyes.The binding stoichiometry was observed as 1:1, and the association constant was calculated to be 1.77×10 5 ML -1 .The limit of detection was calculated to be 3.2×10 -8 M. Logic gate circuit was also developed with the help of fluorescence intensity optical signals.
With the help of a simple process, a Schiff base probe 44 (Figure 13) was developed by Xu and his research group [90] to detect Zn 2+ ions based on imidazo[2,1-b]thiazole-6-carboxylic acid and ethoxy hydroxybenzaldehyde.This probe showed binding capability towards Zn 2+ ions and was investigated by various spectroscopic techniques.This probe shows excellent fluorescence enhancement with Zn 2+ ions in the presence of other metal ions, which shows the enrichment in sensitivity and selectivity towards Zn 2+ ions.The binding stoichiometry was observed to be 1:1 between the probe 44 and Zn 2+ ion, and was confirmed with the help of jobs plot.The association constant was calculated to be 2.22×10 5 ML -1 .The detection limit was 1.2×10 -9 M and the fluorescent signals were utilized in the formation of logic gates.
A turn-on fluorescent probe 45 (Figure 13) has been synthesized for the detection of Zn 2+ ions by Sudipa Mondal and her group.[91] The fluorescent probe was based on sulfaguadine Schiff base.This probe 45 has high selectivity and sensitivity that was investigated by the various photo optical tools or several spectroscopic techniques.The limit of detection was observed to be 37.2 nm that is much lower than the WHO guidelines.The stoichiometry ratio was 2:1 for the probe and Zn 2+ ions in the complex.The complex shows antibacterial activity against grampositive and gram-negative bacteria.
This article is protected by copyright.All rights reserved.With the help of acylpyrazolones and benzhydrazide synthesized, the novel ligands 46 (Figure 14) work Well to detect the Zn 2+ ion by Shaikh et al. [92] the working of ligands investigated with the help of various spectroscopic techniques.This ligand shows anti-malarial activity.

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A new Schiff base probe 47 (Figure 14) has been developed by using route of condensation reaction path by Lijuanchen and research group.[93] This probe shows rapid response towards the Zn 2+ ion.The probe shows enhancement of fluorescence intensity while detecting Zn 2+ ion This article is protected by copyright.All rights reserved.
Manuscript in an aqueous media, color change was also observed from yellow-green to strong green.The limit of detection of the probe was 1.9×10 -8 M. furthermore, a filter paper-based paper stripindicator that could be observed with the naked eye also demonstrated good ability to bind Zn 2+ ion, and the binding ratio was 1:1.
A simple naphthylamide based fluorescent sensor 48 (Figure 14) has been developed for the determination of Zn 2+ ion via Schiff base condensation reaction pathway by Xiang et al. [94] This Probe 48 shows very high response time (30 s) and high selectivity towards the Zn 2+ ions.
The probe also showed remarkable enhancement in fluorescence intensity.The binding ratio of the probe and Zn 2+ ion was found to be 1:1 using the jobs plot and the association constant was calculated to be 1.18×10 5 M. The limit of detection value was found to be 39 µM, which is lower than the permissible limit set by WHO.The plausible mode of binding of the complex was verified by DFT computational studies.
To remove Zn 2+ ions from water, Huizhen Wang and his research group [95] have reported a turn-on fluorescent ligand 49 (Figure 14) for the recognition of Zn 2+ ion.The synthesized ligand has been investigated with the help of several spectroscopic techniques.To analyse the sensitivity and selectivity of the ligand towards the Zn 2+ ion, by using emission spectroscopy.
The intensity of fluorescence was increased with the addition of Zn 2+ ion to the solution.The stoichiometric ratio was found to be 1:1 and detection limit was 6.75×10 -9 M.This probe also utilised to detect Zn 2+ ions in living tumour cells.
This article is protected by copyright.All rights reserved.This article is protected by copyright.All rights reserved.This article is protected by copyright.All rights reserved.bearing the two benzothiazole moieties was found to exhibit the lowest detection limit of 0.00028 µM, anticipating it to be the best lead for the recognition of Zn 2+ ions.Further research will continue in the near future to uncover the colorimetric and fluorometric probes to determine Zn 2+ ions.
significance for human growth and industrial development has recently attracted a lot of attention, necessitating the development of effective techniques for the quick detection of Zn 2+ ions.This review paper emphasizes how effective colorimetric and fluorometric chemosensors Schiff bases could potentially be.Due to their superior ability to detect metal ions, Schiff bases are a crucial family of organic probes.These colorimetric and fluorometric probes are extremely efficient to detect Zn 2+ ions with low response time, economical cost and quick change in color.In addition, most of the Schiff base ligands are biocompatible, which were used in the cell imaging of various living organisms to detect Zn 2+ ions.Schiff base probe 34

Table 1
Comparison of the Schiff base probes for the recognition of Zn 2+ ions with the lowest detection limit.